1. Field of the Invention
The present invention is in the field of medical electronics, and more specifically, relates to a compact, battery-operated data acquisition and storage device to be worn by ambulatory patients to facilitate physiological assessment of the condition of cardiac or hypertensive patients. Although the present invention has no moving parts, it performs all the functions of and replaces the portable tape recorder generally used in the art of ambulatory monitoring.
2. The Prior Art
In ambulatory monitoring as normally practiced, the patient wears a device for measuring or sensing the physiological variables to be monitored, such as ECG waves, EEG waves, blood pressure, heart rate, etc. As practiced in the prior art, the patient also wears a miniaturized portable magnetic tape recorder which records the successive values of the physiological signal obtained from the sensors. After the recording session, which may last twenty-four hours or more, the magnetic tape is returned to the clinic where the tape is played back on suitable equipment so that the signals can be recovered and analyzed. The signals from the playback apparatus are typically applied to the printer-plottor or charter which produces a graphical chart record, as well as numerical data.
Various factors limit the ability of the designers of the tape recorder to reduce its weight and size, these being primarily the amount of continuous data recorded and the necessity of accurate reproduction of continuous analog information such as ECG waveforms from the recording into diagnostic quality results. Even though the recording speed for ECG waveforms is made relatively slow in order to sufficiently compact the data, the tape requires considerable length and space. Sufficient battery must be provided to power the tape-drive motor reliably for the entire duration of the monitoring session, and this adds considerable weight. The tape-drive motor normally includes heavier ferromagnetic elements. Such tape recorders are highly developed and sophisticated instruments designed to do their job with a minimum of weight, but nevertheless, even if not burdened with recording of continuous waveforms over a long recording period, such recorders are complicated and are subject to wear of moving parts. Also, the portable magnetic tape recorder tends to be expensive to produce because it must be carefully engineered to minimize power consumption and to operate extremely quietly. A less expensive acceptable substitute would be highly desirable.
An acceptable substitute for the magnetic tape recorder that could accomplish quality recording while having no moving parts, no noise, and high reliability would be considered a worthwhile advance in the art.
Contemporary playback apparatus used to recover long-term recorded ECG and blood pressure data from magnetic tapes is typically very sophisticated. To save time, the apparatus is made capable of playing back the tape at a higher speed than the speed at which it was recorded and of analyzing the data on the tape as the tape is played back. The operator can select the type of analysis he prefers. The more sophisticated analyzers can recognize the occurrence of waveforms associated with a number of heart disorders and can display the data in ways which facilitate closer study of the more interesting cardiac episodes. A contemporary apparatus for playback and analysis of the recorded tapes is described in U.S. Pat. No. Re. 29,921, reissued Feb. 27, 1979 to Isaac R. Cherry and Donald L. Anderson for "Electrocardiographic Computer."
Clearly, if the magnetic tape could be eliminated, the need for at least the mechanical variable-speed tape playback portion of the analyzer would also be eliminated. Until the present invention, there existed no satisfactory substitute for the magnetic tape and its associated recorder and playback equipment in long-term cardiac monitoring. The present invention changed this situation by demonstrating an advantageous substitute for the magnetic tape system which should show the way for future development of the ambulatory monitoring art.
Prior to summarizing the innovative aspects of the present invention, the equipment with which it will be used will be described briefly. That equipment includes: blood pressure measuring apparatus for ambulatory patients, available under the registered trademark PRESSUROMETER.RTM. II from the assignee of the present invention, Del Mar Avionics, of Irvine, Calif.; a magnetic tape recorder for ambulatory patients known as the Del Mar Avionics Model 447; and an intelligent printer-plotter known as the PRESSUROMETER.RTM. CHARTER, available from the same source. Readers familiar with instruments of these types may choose to turn directly to the section entitled "Summary of the Invention."